Push-pull linear hybrid class H audio amplifiers are often powered by split power supplies that provide positive and negative power supply rails. However, split power supplies typically have poor cross regulation, particularly when the power sources that generate the power supply rails are not equally loaded. For example, if one of the power sources is heavily loaded, but the other power source is lightly loaded then the power sources will not produce power supply rails of the same magnitude. This is typically the result of imperfect coupling of the transformer windings, stray inductance in the transformer and winding resistance.
To compensate for poor cross regulation between the power supply rails, amplifier circuits traditionally include feedback loops that either i) regulate the total voltage of the power supply rails, or ii) regulate either the positive or negative supply rail and hope that the unregulated power supply rail will stay close to the regulated power supply rail.
The problems with these cross regulation compensation methods can be described using the following example. In the exemplary scenario, the audio signal being amplified is a large low frequency sinusoidal signal and at the positive peak of the sinusoid all of the power comes from the first power source. If the total voltage is regulated to 50 V, then the positive power supply rail may be at 20 V and the negative power supply rail may be at −30 V. In this scheme the maximum symmetrical voltage that can be reproduced is only +/−20V.
On the other hand, if the positive supply rail is regulated, for example, even though the positive supply rail may be kept at 25 V through regulation, when the second power source is heavily loaded, the negative supply rail will sag to −20V. Similar to the previous scheme, the maximum symmetrical voltage that can be reproduced is limited to +/−20V.